1. Field of the Invention
The present invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus capable of effectively matching registration between elementary color images.
2. Discussion of the Background
A background full-color image forming apparatus according to an electrophotographic method generally forms toner images of a plurality of elementary colors, such as yellow, magenta, cyan, and black, and sequentially superimposes them on an intermediate transfer member, thereby forming a full-color image on the intermediate transfer member. Then, the full-color image is transferred from the intermediate transfer member onto a recording medium. The apparatus thus performs a full-color image forming. In this operation, however, if the images of the respective elementary colors are not precisely superimposed with respect to the intermediate transfer member, the full-color image formed by the toner images of the elementary colors on the intermediate transfer member has a so-called color drift and is transferred with the color drift onto the recording medium. As a result, quality of an obtained image is deteriorated because of the color drift. Therefore, in order to form a high-quality image, a position control is necessary for aligning start positions of transferring images of the elementary colors on the intermediate transfer member.
There is one attempt to solve this problem. In this case, a full-color image forming apparatus has a structure in which a single photoconductor is employed, and an intermediate transfer belt and a plurality of development devices of different colors are placed to face this photoconductor. With this structure, each time a toner image of a color is formed on the photoconductor, the toner image is transferred to the intermediate transfer belt. Thus, a plurality of toner images are transferred to and superimposed into a full-color image on the intermediate transfer belt, and such a full-color image is transferred to a recoding medium.
This full-color image forming apparatus includes a first reference signal generating mechanism for generating a first reference signal indicating a reference position of the photoconductor, a mark forming mechanism for forming a reference mark according to the first reference signal, and a second reference signal generating mechanism for detecting the reference mark and generating a second reference signal for the intermediate transfer belt. To prevent the above-mentioned color drift in superimposing the color toner images, the full-color image forming apparatus starts transferring an image of a first color on the photoconductor according to the first reference signal, and further transfers images of the second and subsequent colors on the photoconductor based on the second reference signal. This attempt, however, cannot be applied to a full-color image forming apparatus having a structure in which a plurality of photoconductors are employed.
There is another attempt to solve the problem. In this attempt, a full-color image forming apparatus includes a plurality of image forming-mechanisms and an intermediate transfer member placed to face the plurality of image forming mechanisms. Each of the plurality of image forming mechanisms includes an image carrying member, a writing mechanism, at least two development mechanisms for developing an electrostatic latent image formed on the image carrying member by the writing mechanism, and a switching mechanism for alternatively selecting to drive one of the at least two development mechanisms.
The intermediate transfer member includes a non-image region in which a plurality of marks are formed at equal intervals in a rotation direction of the intermediate transfer member. The full-color image forming apparatus further includes a detection mechanism for detecting the plurality of marks, and a PLL (phase-locked loop) circuit to which a signal output from the detection mechanism is input. The PLL circuit outputs a reference signal for starting an image writing performed by the writing mechanism. With this configuration, the full-color image forming apparatus can prevent a jitter of the toner image in a sub-scanning direction and a color drift in the toner images superimposed on the intermediate transfer member. This attempt, however, is required to align the plurality of marks with extremely high accuracy. Further, this structure cannot cope with deterioration over time of the intermediate transfer member and changes of an environment in which the full-color image forming apparatus is used.